Silver

Do you think he is dead? she asked, with seeming irrelevancy. He did not ask her to whom she referred. I believe so. Silver and fire are both deadly to evil spirits, and he got a belly-full of both. - The Black Stranger

Too weak to create effective weapons, silver is primarily for currency, ornamentation and the backing of polished mirrors.

Silver can be effective against demonic creatures and spirits. Many a mercenary always keeps a single silver coin with them to ward off curses and dark creatures.

This metal can be smelted and used to forge weapons but any such weapon will break after anything more than a single hit